The present invention relates to an improved method and apparatus for detecting intrusions and more particularly to a method and apparatus that uses a plurality of sensors. The methods and apparatus of the present invention provide for improved immunity to false alarms.
Intrusion detection systems having a plurality of detectors to improve immunity to false alarms are well known in the art. For example, an intrusion detection system will typically use a passive infrared sensor directed to detect intrusion in a volume of space by sensing infrared radiation, and a microwave detector directed to detect intrusion in the same volume of space by sensing the frequency of reflected microwave radiation in comparison to the frequency of incident microwave radiation. When a signal is simultaneously generated by both of the sensors, signal processing circuitry gates the signals and generates an alarm signal.
Another example of an intrusion detection system employing a plurality of sensors is shown in U.S. Pat. No. 4,853,677 (see also U.S. Pat. No. 4,928,085). There, a single microphone detects both the audible sound of breaking glass and the subsonic sound of pressure on the glass being flexed both before and during breakage. Here again, although a single microphone is used, two different types of physical phenomena are detected (audible sound waves and low frequency pressure waves) to provide a detection system with greater immunity to false alarms.
U.S. Pat. No. 5,107,249 shows an intrusion detection system having a first sensor and a second sensor, with the second sensor being less susceptible to the generation of false alarms than the first sensor. When the second sensor detects an intrusion, the second sensor generates an output signal and this output signal is held. The held output signal is supplied to a logic gate that receives the signal directly from the first sensor. When the first sensor is activated within the period of time that the output signal is held, the logic gate generates an alarm signal. However, this solution is less than ideal because random events that trigger the second sensor will cause the system to become a single technology device for the period of time that the output signal is held. Worse yet, during the period of time that the output signal of the second sensor is held, the system effectively operates as a single technology system that is dependent upon the less reliable technology.
Accordingly, in the present invention, an improved intrusion detection system having a plurality of sensors that is more immune to false alarm generation is disclosed.